Amplifier



May 15, 1945. w. VAN B. ROBERTS AMPLIFIER Filed Jan. 27', 1945 2Sheets-Sheet 1 MODULHTED WA VE SOURCE To mwr/wvn M00. WflVE SOURCE m. EM Z r m .8 V 0 mg A WY 8 W. VAN B. ROBERTS AMPLIFIER Filed Jan.

2 SheetsJSheet 2 MOD1/Lfi TE D WA VE SOURCE IIVVENTOI? W. ww B. ROBE/e7'3 wsawm ATTORNEY Patented May 15, 1945 AMPLIFIER Walter van B.Roberts, Princeton, N. 3., aasignor to Radio Corporation 0! America, acorporation oi Delaware Application January 27, ms. set-ran No. 473,695

c on. (oi. 179-l7l) rangement in accordance with my invention. In

this circuit features known in the art such as neutralizing connectionshave been omitted for the sake of simplicity, The system as illustratedcomprises two stages coupled and arranged and 1 operated in a novelmanner.

Figs. 2 and 3 are modifications oi the arrangement oi Fig. 1.

In the Doherty system (Sept. 1936, IRE page 1163) two tube amplifierstages have their input excited in phase quadrature by alternatingcurrent and their outputs coupled to a common. load R. One stage isbiased to operate at carrier and above (1. e., cut ofi at carrier) whilethe other stage operates at carrier and below and has an impedanceinverting network in its output coupling'to the load. The network feedsinto a load or terminating impedance R but is given a surge impedance2R. At carrier condition, that is when said one tube is cut on, theinput impedance of the network is therefore equal to R.

The last mentioned other tube stage supplies the carrier powereiliciently and feeds through the network into the load resistance 'R'which also is the terminating impedance of the net- 3 work and remainsconstant and equal to R so that tube works intoa network input 01'constant impedance and El the tube stage output voltage varies linearlywith the applied grid voltage, On peaks of applied voltage which exceedcarrier amplitude the said one tube stage (biased to cut off for carrieramplitude) feeds power to the load R thus raising the effectiveterminating imped-, ance of the network thereby lowering the inputimpedance of the network into which the said other mentioned tube works.This other stage as a consequence delivers more power although itsoutput voltage El remains constant. At the peaks or say full modulation.conditions are such that the said one tube stage delivers half of thetotal power thus raising the efiective network termination impedancefrom R to 2R. ohms and lowering the network input impedance into whichthe said one tube works from 4R to 2R. The said one tube then deliverstwice carrier power as does the said other tube. giving a total power offour times the carrier power.

The object of my invention is to improve and simplify amplifiers of thenature described in the preceding paragraph.

The simplified circuit of Fig. 1 illustrates my improved system. In theinterest of simplicity refinements such as neutralizing circuits, etc.,have been omitted.

The stage tube in has its input electrodes coupled to a source ofalternating current H by way of a tuned circuit It. The grid of thistube is biased about to cutofi at carrier amplitude by source It. Theoutput electrodes are connected 5 in tuned output circuit l8 and thepower is fed by a filter 20 to any load such as an antenna 22. The stagetube 26 has its input electrodes connected to a tuned circuit 88 coupledto tuned circuit I i. This tube is biased to operate for impressedvoltage by, for example, resistance 38. The output electrodes of tube 26are connected to a tuned circuit 38 which is coupled to circuit 98.Circuits M, i8, 30 and 38 are tuned to the same frequency, i. e'., thefrequency of the alternating current from source i2,

Assume circuit M is excited by-alternating current of carrier amplitude,or less. The bias in this stage tube it is not overcome so that it doesnot supply plate current to circuit I 8 or total load R. The mutualbetween circuits M and 30, however, is made such that the inputelectrodes of stage tube 26 are .excited equally with the excitation onthe input electrodes of tube iii.

Moreover, this method of coupling causes the 5 excitation voltage on theinput electrodes of tube 28 to be shifted 90 with respect to that on theinput electrodes of tube 10. This phase shift has been shown bymeasurement.

The tube 26 now feeds into circuit 38 and thence into circuit l8 and tothe load impedance which I take as R ohms. Rememberingthat the tube Itis inactive the eilect or the mutual inductance Mp is to throw aresistance proportional to HR across tank circuit 36, i. e., across theplate of tube 25. This is made high enough to make tube 26 work at highefllciency for all voltages of amplitude less than carrier amplitude.

Under these conditions tube 26 operates into a constant impedance withits plate voltage E following its grid excitation voltage and we canassume that this impedance into which tube 28- works is adjusted bymeans of Mp to be 4R.

,Now as the modulation increases the tube i0 cutofi bias is overcome andtube i0 supplies power to the load thus increasing the total effectiveimpedance R. This decreases the load reflected across the circuit 38 andthereby lowers the impedance into which tube 26 is working and the tube28 feeds more power into circuit 38 and thence to the load R.

When peak modulation is reached, 1. e., twice carrier excitation, thearrangement is such that the impedance into which tube 26 works isreduced by about and is 2R and tube- 28 supplies to the loadR twiceasmuch power as at carrier excitation. The arrangement is also such thattube It) is then supplying about half the total power to the load.

As to the inverting action between l8 and 38, it can be shown that theeffective load conductance across 38 is inversely proportional to theload conductance across I8 when the Q's of said circuits aresufliciently large as is the case in practice. By a suitable adjustmentof the mutual Mp therefore the resistance R may be caused to create theeffect of aresistance of IR across 36 while a resistance 2R across l8acts like a resistance of 2R.

.There is also produced a 90 phase shift which offsets the phase shiftbetween the excitation voltages.

In practice the couplings and loading are ad- J'usted by cut and try"until the required opera ating conditions are attained such as equalpower frorln both tubes at the peaks of the modulating cyc e.

The output of the system ,of- Fig. 1 may be supplied to any utilizationmeans such as an antenna 22 by way of a filter circuit 20.

The modification illustrated in .Fig. 2 makes use of the principledescribed above in detail in connection with Fig. 1. Fig. 2 diflers fromFig. l in that a link circuit 40 couples the tuned circuits l4 and 30and provides variable coupling between these circuits whereby equal butsubstantially 90 displaces excitation voltages are fed to the inputelectrodes of tubes 28 and I0. A second link circuit 42 couples thetimed circuits 86 and l8 and these couplings as in'Fig'ure 1 areadjusted so that when taken with the loading provide the properamplitudeproper phase excitation voltages at the input or tube 26 andthe impedance inverting eflect described above between circuit l8 and36. I

In the arrangement of Fig. 3 each. stage comprises a pair of tubesconnected in pushpull rela-v tion with the circuits neutralized. Thesetubes, except for the circuit connections which followfrom the use oftwo tubes instead or one, are in principle the same as in Fig. 1 and theoper-' ation therefore, it is believed, is entirely clear from theforegoing description. In this case also, linkcoupling may be usedbetween input circuits and/or between output circuits.

The advantages of my system are:

1. It requires no additional elements for phase shifting and impedanceinversion as these functions are accomplished by the usual tunedcircuits in the two stages. I 1

2. It is equally well adapted to a pair of pushimll stages.

I claim:

l. A first amplifier stage having a tuned input circuit and a tunedoutput circuit, a second amplifier stage having a tuned input circuitand a tuned output circuit, there being eilective mutual couplingbetween said input circuits such that when one thereof is excited bymodulated voltages to be amplified the other thereof is also excited byvoltages of nearly equal amplitude and if ted about 90, there beingeffective mutual upling between said outputcircuits such that theimpedance across one thereof is substantially proportional to thereciprocal of. the impedance across the other thereof, and means forbiasing one of said amplifiers for operation at carrier output andabove.

2. A systemas recited in claim '1 wherein said couplings comprise a linkcircuit between the input circuits and a link circuit between the outputcircuits.

3. A system as recited in claim 1 wherein said amplifiers are eachneutralized.

4. Asystem as recited in claim 1 wherein said amplifiers are of thepush-pull type.

.5. In an amplitude modulated wave amplifier system, a tube stage havinginput electrodes cou- 'pled by a tuned circuit to a source ofalternating current and having output electrodes coupled by a secondtuned circuit to a load, means for bias-' ing said stage substantiallyto,cut oil in the presence of alternating current of carrier waveamplitude and less, a second tube stage having input and outputelectrodes, a tuned circuit cousaid last named :tuned circuit, a. tunedcircuit coupled with the output electrodes of said last pled to saidinput electrodes, there being mutual inductance between said first namedtuned circuit and said last named tuned circuit, a tuned circuit coupledwith the output electrodes ofsaid second tube, there being mutualinductance between said second tuned circuit and said last named tunedcircuit and means for supplying a bias to the input electrodes of saidsecond named I tube, such that it amplifies small voltages impressedthereon.

6. In an amplitude modulated wave amplifier system, a pushpull amplifiercomprising two tubes having input electrodes coupled by a tuned circuit,to a' source of alternating current and having output electrodescoupled by a second tuned circuit to a load, means for biasing the tubesof said stage substantially .to cut of! in the presence of alternatingcurrent ofcarrier wave amplitude and less, a second push-pull amplifiercomprising twotubes having input and output electrodes, a tuned circuitcoupled with said input electrodes, there. being eflective mutualimpedance between said first named tuned circuit and mentioned twotubes, there being eifective mutual impedance between said second tunedcircuit and said last named tuned circuit and means for supplying a biasto the input electrodes of said second named two tubes, such that theyamplify small voltages impressed thereon; I

v WAL'I'ER VA! 3. aopaa'rs.

